Helicopter lifting and transportation of external loads on a single point or dual point cable system is a high work load operation requiring the pilot's constant attention. A load suspended beneath a helicopter by one or more cables or slings forms a spring-mass system which is easily excited into oscillation in the vertical direction--the pilot may interact with this oscillation in a phenomenon commonly called vertical bounce. The excitation can result from a variety of phenomenon, including wind gusts, air pressure drops, abrupt pilot input, abrupt load pickup, etc. The frequency of vertical oscillation is a complex function of the mass of the load, the characteristics of the cable, the dynamics of the helicopter, and the physiological characteristics of the pilot. Since the pilot's body is subject to oscillation of the aircraft, if his hand is on the collective pitch control stick (as is normally the case), his body motion can cause reinforcement of the oscillations through collective stick inputs to the control system. There is also a natural tendency for the pilot to attempt to compensate for the oscillations--that is, to counteract them with corrective inputs to the collective pitch stick. However, due to human reaction lag and lag in aircraft and load response, such inputs tend to reinforce, rather than cancel, the oscillations.
The vertical bounce mode is totally different from the pendular, horizontal modes controlled by the inner stability loop in the manner described in commonly-owned U.S. Pat. Nos. 3,756,543 and 3,833,189.
In the past, the only solution to the vertical bounce problem was to have the pilot simply take his hand off the collective pitch control stick. This would allow the vertical oscillations to dampen naturally, and die down. However, this is a very unnatural act for a pilot, and could be dangerous during close maneuvering. As a consequence, many loads have been jettisoned so as to save the helicopter.
It has been known to utilize control desensitizer systems in the pitch, roll and yaw axes, to reduce the tendency for pilot interaction with the helicopter's fundamental fuselage bending mode frequencies. The interaction may manifest itself passively as a result of fuselage vibrations being transmitted through the pilot's seat, through his body and arms or legs, to control sticks and pedals. Natural frequencies of on the order of several Hertz (Hz) are common (depending on the craft, the load and the pilot). These interactions have been desensitized by a filtered feedback system that continuously senses high frequency (several Hz) stick and pedal inputs, and cancels them. An example is shown in commonly-owned U.S. Pat. No. 4,279,391 to Adams et al.
The use of a system as in Adams et al has been found to be impractical for desensitizing the vertical bounce mode because the vertical oscillation frequency is so low (on the order of one-half Hz to several Hz) that high-pass filtering of unwanted oscillations can be achieved only with a corresponding reduction in normal aircraft handling quality.
In "Rotor and Wing International", September 1985, at pp. 15-16, it is suggested that the collective pitch stick can be locked with friction or a soft spring on the stick can reduce its natural frequency below two Hz. But these suggestions are not deemed to present adequate solutions.